Wavelength-division multiplexing is a valuable technique for increasing the information capacity of an optical communication network. In a multiplexed network, it is essential to provide wavelength-selective components that can separate a particular wavelength channel from a group of multiplexed channels. It is advantageous to use Bragg gratings to provide spectral selectivity, because they are readily made with passbands that match desired channel widths, and because they are readily integrated with optical fibers. Channel-selective filters using Bragg gratings have, in fact, been reported. However, at least some of these filters suffer from relatively high coupling losses, and others are difficult to manufacture because they must be made to extremely close tolerances.
For example, one well-known filter configuration includes an input fiber coupled to a fiber-based Bragg grating through a directional optical coupler. Light reflected from the grating is coupled to an output waveguide through the same coupler. In this manner, the narrow reflection band of the grating is effectively transformed into a transmission band for the filter as a whole. However, this arrangement suffers 6 dB of coupling loss, to which is added the intrinsic loss in the directional coupler.
Another example is a filter described in K. O. Hill et al., "Narrow-Bandwidth Optical Waveguide Transmission Filters," Electronics Letters 23 (1987) 465-466. This filter consists of a loop of single-mode optical fiber which includes a fused-taper directional coupler at the point where the loop branches off from the straight part of the fiber. A Bragg grating is situated symmetrically midway in the fiber loop. This filter can be made to have relatively low loss. However, this filter is difficult to manufacture because exact placement of the Bragg grating is required in order to achieve the desired transmission characteristics.
Compound Bragg reflection filters made in planar waveguides are described in C. H. Henry et al., "Compound Bragg Reflection Filters Made by Spatial Frequency Doubling Lithography," J. Lightwave Technol. 7 (1989) 1379-1385. As reported therein, a filter having desirable spectral properties can be made by forming a Bragg grating having many, e.g., 15, sections, each with a different spatial period. To avoid phase shifts near 90.degree. between sections, the sections are made contiguous, resulting in a grating period that is piecewise constant, but discontinuously varied. Henry et al. does not teach how to make such a filter in an optical fiber. Moreover, the filter of Henry et al. is relatively complicated to manufacture, and it would still be desirable to provide a filter that is simpler to manufacture.